Abstract:

Disclosed is a fuel for fuel cells which contains at least one organic
compound selected from the group consisting of methanol, ethanol,
dimethyl ether and formic acid, and 1-200 ppm of a hydrocarbon compound
in terms of a single component as determined by gas chromatography mass
spectrometry. Also disclosed are a fuel cartridge for fuel cells and a
fuel cell.

Claims:

1. A fuel for fuel cell, comprising:at least one type of organic compound
selected from the group consisting of methanol, ethanol, dimethylether
and formic acid; anda hydrocarbon-based compound in amount of 1 to 200
ppm in terms of a single component measured by a gas chromatography mass
spectrometry.

2. The fuel for fuel cell, according to claim 1, wherein a concentration
of the hydrocarbon-based compound is 5 to 150 ppm.

3. The fuel for fuel cell, according to claim 1, wherein a concentration
of the hydrocarbon-based compound is 10 to 100 ppm.

5. The fuel for fuel cell, according to claim 1, wherein the at least
organic compound is methanol and a concentration of methanol is 50% by
mole or more.

6. A fuel cartridge for a fuel cell, comprising: a fuel container and a
fuel contained in the container, the fuel contains:at least one type of
organic compound selected from the group consisting of methanol, ethanol,
dimethylether and formic acid, anda hydrocarbon-based compound in amount
of 1 to 200 ppm in terms of a single component measured by a gas
chromatography mass spectrometry.

7. The fuel cartridge according to claim 6, wherein a concentration of the
hydrocarbon-based compound is 5 to 150 ppm.

8. The fuel cartridge according to claim 6, wherein a concentration of the
hydrocarbon-based compound is 10 to 100 ppm.

10. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing low-density polyethylene and
the hydrocarbon-based compound contains ethylene.

11. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing high-density polyethylene or
linear low-density polyethylene and the hydrocarbon-based compound
contains ethylene and α-olefin.

12. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing denatured polyethylene and the
hydrocarbon-based compound contains ethylene, acrylic acid and maleic
anhydride.

13. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing polypropylene and the
hydrocarbon-based compound contains propylene, ethylene and
α-olefin.

14. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing polyamide and the
hydrocarbon-based compound contains hexamethylenediamine, adipic acid,
ε-caprolactam, ω-laurolactam and dodecanoic acid.

15. The fuel cartridge according to claim 6, wherein the fuel container is
formed of a polymeric material containing polyethyleneterephthalate and
the hydrocarbon-based compound contains terephthalic acid, ethyleneglycol
and cyclohexanedimethanol.

16. A fuel cell comprising:a fuel storage portion which stores a fuel
containing at least one type of organic compound selected from the group
consisting of methanol, ethanol, dimethylether and formic acid;a fuel
gasifying portion that allows a gasified component of the fuel to pass
through; anda membrane electrode assembly including a fuel electrode to
which the gasified component is supplied, an oxidizer electrode and a
polymer electrolyte membrane provided between the fuel electrode and the
oxidizer electrode,wherein an amount of a hydrocarbon-based compound
contained in the fuel held in the fuel storage portion, the fuel
gasifying portion and the membrane electrode assembly is 1 to 1500 ppm in
terms of a single component measured by a gas chromatography mass
spectrometry.

17. The fuel cell according to claim 16, wherein a concentration of the
hydrocarbon-based compound is 1 to 200 ppm.

18. The fuel cell according to claim 16, wherein a concentration of the
hydrocarbon-based compound is 5 to 150 ppm.

20. The fuel cell according to claim 16, wherein the organic compound is
methanol and a concentration of methanol is 50% by mole or more.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a fuel for fuel cell, a fuel
cartridge for fuel cell and a fuel cell.

BACKGROUND ART

[0002]Recent years, various electronic devices such as personal computers
and mobile telephones have been reduced in size in accordance with the
advance of the semiconductor technology. As the downsizing of the
electronic devices proceeds, there have been attempts of using fuel cells
as the power for the small-sized devices. The fuel cell can generate
electrical power simply by supplying a fuel and an oxidizer thereto, and
has such an advantage that it can continuously generate the electrical
power by replacing the fuel. Because of such an advantage, the fuel cell,
if it can be reduced in size, is considered to be an extremely useful
system for driving mobile electronic devices. In particular, a direct
methanol fuel cell (DMFC) is a promising power source for small-sized
devices since it uses methanol, which has a high energy density and can
be easily handled as compared to the hydrogen gas fuel, and electrical
current can be obtained from methanol on an electrode catalyst without
requiring any reforming instrument.

[0003]There are several methods of supplying fuel to a DMFC, for example,
a gas-supply type DMFC in which liquid fuel is gasified and fed into the
fuel cell with a blower or the like, a liquid-supply DMFC in which liquid
fuel is fed into the fuel cell with a pump, and an internal gasifying
type DMFC in which liquid fuel is gasified within the fuel cell. Of
these, the internal gasifying type DMFC requires no large-scale
equipments such as pump or blower for supplying the fuel. Therefore, the
concentration of the liquid fuel is increased and the liquid fuel tank
can be reduced in size, it becomes possible to achieve the realization of
a small-sized fuel cell having a high energy density.

[0004]It should be pointed out here that Jpn. Pat. Appln. KOKAI
Publication No. 2004-311163 discloses an improvement of the performance
of a cell, which can be achieved by regulating the density of an organic
compound component (formic acid, acetic acid or oxalic acid) contained in
a catalyst layer of an electrode of the fuel cell.

DISCLOSURE OF INVENTION

[0005]An object of the present invention is to provide a fuel for a fuel
cell, a fuel cartridge for a fuel cell and a fuel cell, which can improve
the stability in generation of electric power for a long time.

[0006]According to an aspect of the present invention, there is provided a
fuel for a fuel cell, comprising: at least one type of organic compound
selected from the group consisting of methanol, ethanol, dimethylether
and formic acid; and a hydrocarbon-based compound in amount of 1 to 200
ppm in terms of a single component measured by a gas chromatography mass
spectrometry.

[0007]According to another aspect of the present invention, there is
provided a fuel cartridge for a fuel cell, comprising: a fuel container
and a fuel contained in the container, the fuel contains at least one
type of organic compound selected from the group consisting of methanol,
ethanol, dimethylether and formic acid, and a hydrocarbon-based compound
in amount of 1 to 200 ppm in terms of a single component measured by a
gas chromatography mass spectrometry.

[0008]According to still another aspect of the present invention, there is
provided a fuel cell comprising:

[0009]a fuel storage portion which stores a fuel containing at least one
type of organic compound selected from the group consisting of methanol,
ethanol, dimethylether and formic acid;

[0010]a fuel gasifying portion that allows a gasified component of the
fuel to pass through; and

[0011]a membrane electrode assembly including a fuel electrode to which
the gasified component is supplied, an oxidizer electrode and a polymer
electrolyte membrane provided between the fuel electrode and the oxidizer
electrode,

[0012]wherein an amount of a hydrocarbon-based compound contained in the
fuel held in the fuel storage portion, the fuel gasifying portion and the
membrane electrode assembly is 1 to 1500 ppm in terms of a single
component measured by a gas chromatography mass spectrometry.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 is a diagram conceptually showing a fuel cell according to an
embodiment of the present invention;

[0014]FIG. 2 is a chart diagram showing results of gas chromatography mass
spectrometry of a liquid fuel for the fuel cell of Example 1;

[0015]FIG. 3 is a chart diagram showing results of gas chromatography mass
spectrometry of a liquid fuel for the fuel cell of Example 4;

[0016]FIG. 4 is a characteristic diagram showing a relationship between a
current density and an output density in the fuel cell of each of
Examples 1, 5 and 6;

[0017]FIG. 5 is a characteristic diagram showing a change in current value
along with time in the fuel cell of each of Examples 1, 2, and 4 to 6;
and

[0018]FIG. 6 is a characteristic diagram showing a relationship between a
total amount of hydrocarbon-based compound and an output retention rate
after long-term test in the fuel cell of each of Examples 1, 2, 3, 6 and
7.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019]First, the liquid fuel for a fuel cell will now be described.

[0020]The fuel component of the liquid fuel contains at least one type of
organic compound selected from the group consisting of methanol, ethanol,
dimethylether and formic acid. The fuel component may be of a liquid
containing an organic compound(s) or may be an aqueous solution of an
organic compound(s). For example, in the case where methanol is selected
as the fuel component, it is preferable that the methanol concentration
in the liquid fuel should be 50% by mole or more. A more preferable range
is a concentration exceeding 50% by mole, and most preferably, pure
methanol should be used. With this structure, the container for the
liquid fuel can be reduced in size, and further the energy density can be
increased. The purity of pure methanol should preferably be set to 95% by
weight or more and 100% by weight or less.

[0021]The liquid fuel contains a hydrocarbon-based compound in amount of 1
to 200 ppm in terms of a single component measured by a gas
chromatography mass spectrometry.

[0022]The conditions for the gas chromatography mass spectrometry (GC-MS)
will be described. As the spectrometric column, DB-WAX (30 m×0.25
mmΦ) or an instrument that has functions equivalent to those of this
device can be used. The injection temperature is set to 220° C.
and the column temperature is raised from 50° C. to 220° C.
in the spectrometry. The temperature increasing rate is set to one in a
range of 4 to 7° C. per minute. The detection is carried out in
the total ion chromatogram of the mass spectroscope under the condition
that the scanning mass number is 45 to 425.

[0023]In the GC-MS, when the abundance ratio of a particular component is
50% or more, the concentration is calculated out assuming that the
hydrocarbon-based compound is a single component made of the particular
component. It should be noted that the abundance ratio is calculated from
the peak intensity of the GC-MS.

[0024]In the case where the fuel is made of a number of components where
any one of them does not have an abundance ratio of 50% or more, the
concentration is calculated out supposing that the hydrocarbon-based
compound is C20H40 (icosane).

[0025]The reason why the concentration of the hydrocarbon-based compound
is defined within the above-described range will now be described.

[0026]When the concentration of the organic compound in the liquid fuel is
increased, the low-molecular weight hydrocarbon-based compound (such as
monomer, degradated product of polymer or additive for polymer) becomes
easily elutable from the polymeric part material (such as fuel cartridge)
of the fuel cell. The eluted hydrocarbon-based compound is accumulated on
the membrane electrode assembly (MEA), thereby causing an increase in
resistance and clogging of the gas diffusion layer of the electrode. The
inventors of the present invention carried out intensive studies on the
above-described subject, and found that if a very small amount of the
hydrocarbon-based compound is added rather than being completed
eliminated, the elution is suppressed from the time of the addition. They
further found that when the concentration is 1 ppm or more, the long-term
stability can be improved; however if the concentration exceeds 200 ppm,
a high output may not be obtained due to an increase in resistance and a
decrease in gas diffusibility.

[0027]Therefore, when the concentration of hydrocarbon-based compound is
set in a range of 1 ppm or more but 200 ppm or less, the long-term
stability can be improved while maintaining a high output. A preferable
range is 5 to 150 ppm, and a more preferable range is 10 to 100 ppm.

[0028]Examples of the hydrocarbon-based compound are a monomer, a
degradated product of polymer and an additive for polymer. There may be
one or more types of hydrocarbon-based compounds.

[0045]The type of hydrocarbon-based compound eluted into the liquid fuel
greatly depends on the material for the container of the fuel cartridge.
Types of hydrocarbon-based compounds (monomers) which are easily eluted
will now be indicated in groups of types of the fuel cartridge.

[0052]Examples of the fuel cell that uses the above-described liquid fuel
or fuel cartridge are a liquid fuel-supply type cell and an internal
gasifying-type cell. The internal gasifying type cell has such a
structure that a gasified component of a liquid fuel is supplied to a
fuel electrode, and therefore it is desired that a high-concentration
liquid fuel should be used in order to assure a sufficient amount of
gasifying. Further, since the hydrocarbon-based compound in the liquid
fuel is concentrated due to the gasifying, the problem of clogging of the
fuel gasifying unit may be created. Thus, when the amount of the
hydrocarbon-based compound is regulated in the internal gasifying-type
fuel cell, a sufficient improvement in performance can be achieved.

[0054]As shown in FIG. 1, the membrane electrode assembly (MEA) includes a
proton-conductive polymer electrolyte membrane 1, an air electrode
(oxidizer electrode) 2 formed on one surface of the electrolyte membrane
1 and a fuel electrode 3 formed on an opposite surface of the electrolyte
membrane 1.

[0055]It is preferable that the polymer electrolyte membrane should
contain a proton-conductive material as a main component. Examples of the
proton-conductive material are a fluorine-based resin having a sulfonic
acid group (such as a perfluorosulfonic acid polymer), a
hydrocarbon-based resin having a sulfonic acid group and inorganic
materials such as tungstic acid and phosphorus tungstate; however the
material is not limited to these examples.

[0056]Each of the air electrode 2 and fuel electrode 3 is equipped with a
catalytic layer and a gas diffusion layer. Examples of the catalyst
contained in the catalyst layer are platinum metal elements (such as Pt,
Ru, Rh, Ir, Os and Pd) and an alloy containing a platinum metal element.
It is preferable that Pt--Ru, which has a strong resistance to methanol
or carbon monoxide should be used as the fuel electrode catalyst, whereas
platinum should be used as the air electrode catalyst; however the
catalysts are not limited to these materials. It is alternatively
possible to use a supported catalyst that uses an electro-conductive
supporter such as a carbon material, or it may be a non-supported
catalyst. As the gas diffusion layer, carbon paper, for example, can be
used.

[0057]As the fuel storage portion 4, a fuel cartridge, for example, can be
used. A fuel outlet of the fuel cartridge is connected to the fuel
gasifying portion.

[0058]A gas-liquid separation membrane serving as the fuel gasifying
portion 5 allows only the gasified component of the liquid fuel to pass
therethrough, and thus the liquid component of the fuel cannot pass
therefore. It should be noted reference numeral 6 denotes an external
circuit.

[0059]An electric power generating reaction in the case where methanol is
used as a fuel in a fuel cell having the above-described structure will
now be described. The gasified component of the liquid fuel supplied from
the fuel outlet of the fuel cartridge 4 is allowed to pass through the
gas-liquid separation membrane and guided to the fuel electrode 3. At the
fuel electrode 3, a catalytic reaction represented by the following
reaction formula (1) occurs.

CH3OH+H2O→CO2+6H++6e.sup.- (1)

[0060]It should be noted that water used in the reaction formula (1) may
be supplied from the liquid fuel or water in the polymer electrolyte
membrane 1 can be used.

[0061]Protons (H+) generated from the reaction represented in the
formula (1) are allowed to pass through the polymer electrolyte membrane
1 and supplied to the air electrode 2. Electrons flow through the
external circuit 6 to the air electrode 2. The air, which serves as an
oxidizing agent, is taken from an external environment. At the air
electrode 2, the catalytic reaction represented in the following reaction
formula (2), that is, the electric power generating reaction, occurs.

(3/2)O2+6H++6e.sup.-→3H2O (2)

[0062]An overall reaction formula of the catalytic reactions (1) and (2)
is represented in the following formula (3).

CH3OH+(3/2)O2→CO2+H2O (3)

[0063]A fuel cell such as described above exhibits an improved long-term
stability while maintaining a high output when the amount of the
hydrocarbon-based compound in the fuel held in the liquid fuel storage
portion 4, the fuel gasifying portion 5 and the membrane electrode
assembly is regulated to 1 to 1500 ppm in terms of a single component
measured by a gas chromatography mass spectrometry.

[0064]How to measure the concentration of the hydrocarbon-based compound
will now be explained.

[0065]The liquid fuel component present in each of the liquid fuel storage
portion, the fuel gasifying portion and the membrane electrode assembly
can be collected with a microsyringe or the like, and the collected
liquid fuel component can be subjected to the analysis. In order to
extract the fuel component impregnated in these members, the liquid fuel
storage portion, the fuel gasifying portion and the membrane electrode
assembly are immersed in a type of methanol used in precise analysis, for
several hours at room temperature (a smallest possible amount of
methanol, for example, about 5 to 10 ml), and thus the component is
filtrated out. Thus, the fuel components collected and extracted from the
liquid fuel storage portion, the fuel gasifying portion and the membrane
electrode assembly are gathered all together to prepare a sample, which
is subjected to the gas chromatography mass spectrometry. With regard to
the gas chromatography mass spectrometry and the single component
conversion, refer to the descriptions of these provided before.

[0066]Examples of the present invention will now be described in detail
with reference to accompanying drawings.

[0071]A perfluorocarbon sulfonic acid membrane (Nafion film of Du Pont)
having a water content of 10 to 20% by weight and serving as a proton
conductive electrolyte membrane was placed between the anode catalyst
layer and cathode catalyst layer, and they were subjected to hot press,
and thus a membrane electrode assembly (MEA) was obtained.

[0073]Methanol containing 10 ppm of the hydrocarbon-based compound
measured in terms of C20H40 by the gas chromatography mass
spectrometry and having a purity of 99.9% by weight was contained in a
fuel cartridge having a liquid fuel container made of a low-density
polyethylene (LDPE). The results of the gas chromatography mass
spectrometry of the liquid fuel used in Example 1 are shown in FIG. 2. In
FIG. 2, the horizontal axis indicates the time and the vertical axis
indicates the abundance. As shown in this figure, the hydrocarbon-based
compound was a type that contains degraded products of ethylene monomer,
dimer of ethylene, α-olefin monomer and polyethylene and has 10 to
30 carbons.

[0074]The conditions for the gas chromatography mass spectrometry were as
follow. As the spectrometric column, DB-WAX (30 m×0.25 mmΦ) was
used. The injection temperature was set to 220° C. and the column
temperature was raised from 50° C. to 220° C. in the
spectrometry. The temperature increasing rate was set to one in a range
of 6° C. per minute. The detection was carried out in the total
ion chromatogram of the mass spectroscope under the condition that the
scanning mass number was set to 45 to 425.

[0075]With the obtained membrane electrode assembly, the gas-liquid
separation film and the fuel cartridge, an internal gasifying type direct
methanol fuel cell having a structure as shown in FIG. 1 was assembled.

Example 2

[0076]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
concentration of the hydrocarbon-based compound in the liquid fuel was
set to 50 ppm.

Example 3

[0077]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
concentration of the hydrocarbon-based compound in the liquid fuel was
set to 100 ppm.

Example 4

[0078]Methanol containing 50 ppm of the hydrocarbon-based compound
measured in terms of dimethylterephthalate (DMT) by the gas
chromatography mass spectrometry and having a purity of 99.9% by weight
was contained in a fuel cartridge having a liquid fuel container made of
a non-drawn material of polyethyleneterephthalate (PET) denatured by
1,4-cyclohexandimethanol. The results of the gas chromatography mass
spectrometry of the liquid fuel used in Example 4 are shown in FIG. 3. In
FIG. 3, the horizontal axis indicates the time and the vertical axis
indicates the abundance. As shown in this figure, the hydrocarbon-based
compound was a type that contains dimethylterephthalate (DMT), ethylene
glycol and cyclohexanedimethanol. The abundance of DMT in the
hydrocarbon-based compound was 50% or more.

[0079]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
above-described fuel cartridge was employed.

Example 5

[0080]Methanol containing 0.5 ppm of the hydrocarbon-based compound
measured in terms of C20H40 by the gas chromatography mass
spectrometry and having a purity of 99.95% by weight was contained in a
fuel cartridge having a liquid fuel container made of a linear
low-density polyethylene (LLDPE).

[0081]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
above-described fuel cartridge was employed.

Example 6

[0082]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
concentration of the hydrocarbon-based compound in the liquid fuel was
set to 205 ppm.

Example 7

[0083]An internal gasifying type direct methanol fuel cell having a
structure similar to that of Example 1 was assembled except that the
concentration of the hydrocarbon-based compound in the liquid fuel was
set to 1505 ppm.

[0084]The fuel cells obtained in Examples 1, 5 and 6 were measured in
terms of the change in output density when the current density was
increased, and the results were summarized in FIG. 4. In FIG. 4, the
horizontal axis indicates the current density (mA/cm2) and the
vertical axis indicates the output density (mW/cm2).

[0085]As is clear from FIG. 4, the fuel cell of Example 1 in which the
liquid fuel having a concentration of the hydrocarbon-based compound of 1
to 200 ppm was employed exhibited a higher peak in output density than
that of Example 5 (the concentration of the hydrocarbon-based compound
being less than 1 ppm) or than that of Example 6 (the concentration of
the hydrocarbon-based compound exceeding 200 ppm). Further, the cell of
Example 1 showed a peak in output density at a higher current density
than those of Examples 5 and 6.

[0086]With regard to the fuel cells of Examples 1, 2 and 4 to 7, the
change in current value along with time was measured in order to examine
the effect on the long term performance by impurities in the fuel. The
results were summarized in FIG. 5. In FIG. 5, the horizontal axis
indicates the test time and the vertical axis indicates the current
value. It should be noted that Example 7 in which the liquid fuel having
a concentration of the hydrocarbon-based compound exceeding 1500 ppm was
employed had a low initial current value as compared to those of the
other examples, and therefore it was not measured in terms of the change
in current value along with time.

[0087]As is clear from FIG. 5, the fuel cell of each of Examples 1, 2, 4
and 6 in which the liquid fuel having a concentration of the
hydrocarbon-based compound of 1 to 1500 ppm was employed exhibited a
lower initial current value than that of Example 5, but the decrease in
current value during the test was gentle. Especially, the fuel cells of
Examples 1, 2 and 4 in which the liquid fuel having a concentration of
the hydrocarbon-based compound of 1 to 200 ppm was employed was able to
maintain a high current value during the tests as compared to the case of
Example 6 in which the concentration of the hydrocarbon-based compound
exceeded 200 ppm.

[0088]By contrast, the fuel cell of Example 5 in which the liquid fuel
having a concentration of the hydrocarbon-based compound of less than 1
ppm was employed showed an abrupt current drop even from the beginning of
the test, and the current became lower than those of Examples 1, 2, 4 and
6 in the middle of the test.

[0089]After the performance tests shown in FIGS. 4 and 5, the fuel cells
of Examples 1 to 7 were measured in terms of the concentration of the
hydrocarbon-based compound held in the fuel cartridge, liquid-gas
separation membrane and membrane electrode assembly under the
before-mentioned conditions. The results were: 10 ppm in Example 1, 47
ppm in Example 2, 97 ppm in Example 3, 46 ppm in Example 4, 0.5 ppm in
Example 5, 202 ppm in Example 6 and 1502 ppm in Example 7.

[0090]With regard to the fuel cells of Examples 1, 2, 3, 6 and 7, the
output of the cell after being driven continuously for 1000 hours was
measured and the output retention rate (%) in the long term test was
calculated out (expressed with reference to the initial output being
100%). The results were as shown in FIG. 6. In FIG. 6, the horizontal
axis indicates the concentration (ppm) of the hydrocarbon-based compound
held in the fuel cartridge, liquid-gas separation membrane and membrane
electrode assembly, and the vertical axis indicates the output retention
rate (%) in the long term text.

[0091]As is clear from FIG. 6, the fuel cell of each of Examples 1 to 3
and 6 in which the concentration of the hydrocarbon-based compound in the
fuel cell was 1 ppm or higher but 1500 ppm or less was employed exhibited
an excellent output retention rate in the long term test as compared to
that of Example 7, in which the concentration of the hydrocarbon-based
compound in the fuel cell exceeded 1500 ppm. From the results of FIG. 6,
it can be further understood that as the concentration of the
hydrocarbon-based compound in the fuel cell is lower, a higher output
retention rate in the long term test can be obtained.

INDUSTRIAL APPLICABILITY

[0092]According to the present invention, there is provided a fuel for a
fuel cell, a fuel cartridge for a fuel cell and a fuel cell, which can
improve the stability in generation of electric power for a long time.